Council estates were all nice when they started, after slum clearance. They had a random cross section of good and bad in the same mix as the UK. However, we now have mobility, and an obscession with buying property. So the mix changes from 20 or 40 years back. If a council estate is still quiet and safe, with little traffic, it will still be desirable, and 'nice' people will be offered housing. If it's a dump, because the traffic is heavy - so most people with half a brain have left, it will be used as a sink for idiots who the council are obliged to assist with housing.

The problems are caused by choice and freedom, just as with schools, hospitals etc.

There should be entropy in all these systems, not artificial selection and polarisation